Abstract
Forest classifications by disturbance permit designation of multiple types of both old growth forests and shorter-lived forests, which auto-replace under severe disturbance, and also identification of loss of the disturbance type and associated forest. Historically, fire and flooding disturbance regimes, or conversely, infrequent disturbance, produced unique forests such as disturbance-independent forests of American beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), and sugar maple (Acer saccharum) in the Eastern United States. However, disturbance has changed to primarily frequent mechanical overstory disturbance, resulting in novel forests. To demonstrate the transition to no-analog forests after disturbance change, I compared historical tree surveys (ca. 1837 to 1857) to current surveys in the Northern Lower Peninsula of Michigan. To establish widespread disturbance change effects, I also located where beech and hemlock are currently most abundant throughout the Eastern US compared to historical distribution of beech–hemlock forests. In the Northern Lower Peninsula of Michigan, beech and hemlock historically were about 30% of all trees, but currently, beech and hemlock are 2% of all trees. Red maple (Acer rubrum) increased from 1% to 11% of all trees and aspen (Populus) increased from 2% to 13% of all trees. The squared-chord difference between historical and current surveys was 0.40, or dissimilar forests. Areas with ≥20% beech and hemlock or ≥15% of either species decreased from about 52 million to 6 million hectares, with current distribution restricted to the Northeastern US. Current forests are dissimilar to historical forests, and this transition appears to be driven by disturbance regimes without historical analogs. Disturbance change may provide perspective in forest management for climate change.
Highlights
Received: 18 December 2020Disturbance generates a system to differentiate types of old growth forest that persist under minimal or low severity disturbance and shorter-lived forests that auto-replace under severe disturbance (Figure 1)
Current forests are dissimilar to historical forests, and this transition appears to be driven by disturbance regimes without historical analogs
The effects of Euro-American settlement and altered disturbance change are registered in the environmental records of disturbance-independent forests in Michigan, with similar large-scale effects on disturbance-independent forests in the Eastern United States
Summary
Received: 18 December 2020Disturbance generates a system to differentiate types of old growth forest that persist under minimal or low severity disturbance and shorter-lived forests that auto-replace under severe disturbance (Figure 1). Archetypical closed old growth forests are relatively unaffected both by understory and overstory disturbance and instead, dominant tree species are long-lived and shade-tolerant, while their structure is complex due to the extended time since the last disturbance. Stand-scale replacement occurs after centuries to millennia and is likely to be patchy even . American beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), and sugar maple (Acer saccharum) represent disturbance-independent upland and even wetland forests, according to limited disturbance, shade tolerance, longevity, and complex internal structure [1]. Surface fires did not occur to remove small diameter trees, but larger diameter trees controlled the growing space for long intervals, which overall caused either mortality or suspension of growth in tree regeneration. Small-scale disturbance of individual larger trees (“gap dynamics”) allowed recruitment of shade-tolerant tree species.
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